The present invention relates to paper machines and in particular to supercalenders which are used in connection with paper machines.
Thus, the present invention relates to supercalenders of paper machines utilized for burnishing the paper, such calenders including a plurality of hard or metal rolls and a plurality of soft or filled rolls, the number of which may, for example, be substantially equal to the number of metal rolls. The filled rolls are, for example, paper-coated rolls. Thus, as is well known, the filled rolls are made of a material possessing resilient or elastic properties. The filling of the filled rolls is obtained by compressing fabrics, papers, or nonwoven mats of cellulose fiber under high pressure.
The surface of a paper web manufactured by a paper machine is always more or less uneven and rough after the drying operations have been completed. This unevenness and roughness of the paper surface results from the method by which the web is produced at the wet end of the machine. A wet web formed on a planar wire always has an uneven top surface while its lower surface usually retains a distinct pattern or marking from the wire fabric. The press felts which engage the web also provide at the surface thereof a pattern consistent with the fabric structure of the felts. In addition the lack of evenness of the paper surface results from shrinkage of the paper web as well as from wrinkling thereof during the drying operations.
It is of course well known that the smoothness and gloss of the paper surface are achieved by way of a treatment which takes place after the drying operations, this treatment being carried out by way of a calender. Such a calender will normally include a stack of rolls which are supported for rotation and situated one above the other in a suitable vertical frame with the rolls resting against each other while the web which is treated is conducted through pressure zones or nips defined between the stack of rolls, the web usually travelling from the top toward the bottom of the stack.
The smoothing and burnishing action resulting from the calendering is mainly the result of the compressing and deformation action to which the paper web is subjected in the nips between the calender rolls. In addition, between the web and the surface of the calender rolls there is a greater or lesser differential velocity or slip, producing a friction which promotes the burnishing of the paper, this friction acting on the paper in addition to the effect of the compression of the paper at the nips between the rolls.
The purpose of calendering is to influence even additional paper properties, which is to say in addition to the smoothness and the gloss of the paper surface. Thus, one of the important functions of calendering is to impart to the paper a predetermined thickness or caliper, and in this connection the calendering action should eliminate thickness variations, if any. During treatment in a calender, the thickness of the paper web is reduced, and in a corresponding manner the density of the paper is increased while at the same time the paper web becomes more plastic or pliable.
As is known, calendering is carried out either with a paper machine calender situated in the paper machine between the drying cylinder section and the reeling apparatus, or the calendering is carried out by way of a supercalender which in the past has been in the form of a separate unit utilized for treating paper subsequent to manufacture thereof at a paper machine proper.
The machine calender forms an essential component of most machines for manufacturing paper or cardboard. All of the rolls of such calender are metal rolls made of die-cast steel, for example, and the primary task of such machine calenders, for example in a newsprint machine, is not at all to provide a hard gloss but rather to compress the dry paper web so that it will have a uniform thickness and smoothness over the entire width of the paper web. In a cardboard machine also the primary task of the machine calender is to compress the cardboard web to a predetermined thickness or caliper, while the cardboard also may be burnished on one or both sides. Relatively large cardboard machines require two and in some cases even three consecutive calenders.
Supercalenders, however, are only suitable for burnishing and compacting the paper web. The rolls of supercalenders are alternately metal rolls, such as hard, ground steel rolls, and filled rolls such as, for example, papercoated rolls having a resilient surface. A supercalender may have up to 20 rolls. Moreover, supercalenders are capable of being easily damaged and cannot be operated directly at and connected to the paper machine. Such supercalenders operate separately from the paper machine.
Calendering is particularly necessary in connection with the manufacture of writing and printing papers. In the case of writing papers and papers primarily intended to have text printed thereon, a machine calender treatment may be adequate. If the printing work includes accurate printing of illustrations or printing in several colors, then a high degree of surface smoothness is required for the paper as well as a glossy surface at the same time. In this case smoothing and burnishing treatment by way of a supercalender is essential.
The effect of calendering on a web is dependent upon a large number of factors which include the number of nips, the temperature of the rolls, the moisture content of the paper and the moisture distribution in the web cross section, the machine speed, and the pressure per unit area in the press nip. This latter factor is dependent upon the line pressure, the diameter of the rolls, and the thickness of the web.
The line pressure in a calender must not exceed an empirically determined limit for each particular type of paper. In the event that the line pressure is excessive, the web may be crushed or otherwise damaged and spoiled. In order to avoid this latter drawback and in order to achieve a calendering of adequate efficiency, a very large number of nips is required in certain cases. In a conventional calender where the rolls are usually situated so as to rest freely one upon the other, certain problems of construction and paper technology are taken care of. Thus the nip loading will increase naturally from the top toward the bottom, consistent with the weight of the rolls and their bearings. This will result in deflection of the lower rolls and in an excessive increase of the line pressure which must be eliminated by way of special roll constructions and special arrangements.
It is important that the line pressure at each nip of the calender be as uniform as possible across the web. In order to achieve this latter uniformity in the line pressure it is known to compensate for deflection of the rolls by way of crowning or bulging at the roll surface, brought about by grinding the rolls so that they are somewhat of a slightly barrel shaped configuration. In other words the diameter of such a roll gradually increases from the end toward the center of the roll. The deflection of a roll under different loading conditions can be calculated from the dimensions of the roll, the load forces acting thereon, the density of the roll material, and its modulus of elasticity. In a multiple roll calender the crowning or bulging problem is of considerably greater complexity than in a press made up of only one pair of rolls.
In order to reduce problems in connection with bulging or crowning of the rolls, it is possible to utilize in a calender specially designed rolls which are compensated with respect to deflection and the deflection of which may be regulated so as to be adapted for a particular load. It is also possible to provide suitable weight-relief means for the rolls.
In conventional calender apparatus, there are a number of drawbacks and deficiencies, which include, among others, the following:
It is not possible to achieve a high gloss with the machine calender. Thus, for this purpose a separate supercalender is required, and such a separate supercalender of necessity occupies an undesirably large amount of space and carries with it high construction costs.
The care and operation of a supercalender necessitates a team of its own, so that high labor costs are involved.
The starting up of a supercalender either at the beginning of a calendering operation or in the event of a web break is cumbersome and cannot be carried out at high speed.
Control of uniform line pressure at the calender nips is cumbersome, particularly in connection with calenders which have a large number of rolls. As a result such structures utilize several complex arrangements in connection with loading of the calender rolls, and a large number of expensive deflection-compensated rolls are required.
The roll stack of a conventional supercalender is arranged so that all of the rolls, namely the metal as well as the filled rolls, are situated vertically one above with the other with their axes in a common vertical plane. In the event that such a calender is stopped for a prolonged interval, it is essential to see to it that the resilient surfaces of the filled rolls do not remain under pressure since they may become permanently damaged in this way. Thus, in order to take care of this latter problem, a load-relief system is required.
A further problem encountered during operation of a conventional calender is in connection with the production of so-called lash markings on the paper. Such markings are caused by vibration of the calender, such vibration producing a variation in the line pressure at the nips. The generation of such vibrations can be understood by imagining the calender as a system of mass elements and elastic elements wherein the calender rolls represent the mass elements while the paper at the nips between the rolls represents the elastic spring structure between these masses. The smaller the number of rolls which are stacked one above the other in the calender stack or the greater the diameter of the intermediate rolls which are utilized, the smaller is the likelihood of vibration in the calender.